Process for the electrolytic production of fluorine



Sept. 19, 196 A. DAVIES ET AL PROCESS FOR THE ELECTROLYTIC PRODUCTION OF FLUORINE Filed July 15, 1959 States The present invention relates to improvements in or relating to a process forthe electrolytic production of fluorine and apparatus therefor.

More particularly the present invention relates to improvements in or relating to a process for the production of fluorine by electrolysis at a temperature of 80-110 C. from a fused substantially dry mixture of potassium fluoride and hydrogen fluoride having a composition approximating substantially to KF.1.8 HF to KF.2.2HF with no evolution of fluorine as free bubbles at a substantially vertical carbon anode in conjunction with a gas impermeable barrier which entirely surrounds the upper part of the anode but is not in contact with the anode and in a cell in which the body is of electro-con ducting material and in which the upper portion of the anode is above the level of the top portion of the cathode and to apparatus therefor.

The object of the present invention is to provide a process of the aforesaid kind and apparatus therefor which will permit a cell of the aforesaid kind to run at higher loads, thus to obtain a larger output of fluorine per unit of plant and furthermore fluorine of reduced hydrofluoric acid content,

It has now been found that it is possible to increase the load (in an electrolytic process of the aforesaid kind) and thereby to diminish the concentration of hydrofluoric acid in the fluorine evolved if the cathode was a construction for a cooling medium to be circulated therethrough and if loss of heat is permitted to occur from the walls of the cell, as for instance by water-cooling said walls.

According to the present invention the process for the production of fluorine by electrolysis at a temperature of 80-110" C., preferably 8085 C., from a fused electrolyte comprising a substantially dry mixture of potassium fluoride and hydrogen fluoride having ascomposition approximating substantially to KF.1.8HF to KF.2.2HF with no evolution of fluorine as free bubbles at a substantially vertical carbon anode in conjunction with a gas impermeable barrier which entirely surrounds but is not in contact with the anode and in a cell in which the body is of electro-conducting material, in which the upper portion of the anode is above the level of the top portion of the cathode and in which the cathode is preferably in metallic electrical connection with said body comprises employing a cathode which permits a cooling medium, preferably water, to be circulated therethrough and while electrolysis is taking place circulating a cooling medium therethrough. Y

To make sure that said barrier is not in contact with the anode it is sometimes desirable to provide insulating material in the space within the electrolyte between at least the lower extremity of the barrier and the anode.

In co-pending application Ser. No. 784,151, filed December 31, 1958, it is stated that it has been found, most surprisingly, that it is possible safely, reliably and economically to work a fluorine cell with a substantially vertical solid carbon anode of substantially uniform cross section and barrier of simple robust conventional design and to employ current densities up to for instance 1.1 ampere per square inch using an anode/cathode separation much less than those which have hitherto been atent 3,000,80l Patented Sept. 19, 1961 thoughtindispensible to safety if inter alia said anode had a permeability, for instance, between 1.0 and 30, permeability being here defined in terms of cubic feet of air per square foot of surface capable of passing through one inch thickness of the anode material per minute under an imposed pressure equivalent to 2 inches of water. It is stated therein that the determination of the permeability is carried out on cylinders 1 inch diameter and 1 inch long and that these cylinders are mounted tightly in a rubber holder and that the mean of the measurements of the quantity of air passing through two cylinders which are cut at right angles to one another from the same block, under an imposed pressure equivalent to 2 inches, of water, is used to calculate the permeability. It is also stated therein that ordinary electrode carbon measured in this way has a permeability of 0.05 and is unsuitable for the carrying out of the process claimed therein and that electrode carbon of such low permeability is considered to be impermeable for the purposes of the invention claimed therein. 7

Furthermore, in said co-pending application Ser. No. 784,151 there is claimed inter alia a process for the production of fluorine by electrolysis at a temperature of 80- 110 C., preferably 8085 C., from a fused substantially dry mixture of potassium fluoride and hydrogen fluoride having a composition approximating substantially to KF.1.8HF to KF.2.2HF under non-polarization conditions and so that the composition of the mixture is maintained substantially at KF.1.8HF to KF.2.2HF at a cathodic current efficiency of greater than 90% and preferably greater than 95%, with no evolution of fluorine as free bubbles at a substantially vertical solid gas permeable carbon anode of substantially uniform cross section in conjunction with a gas impermeable barrier which entirely surrounds but is not in contact with the anode by employing curent densities of, for example, 0.1 to 1.1 amperes per square inch and for instance effective lengths of anode and cathode up to for example 18 inches and in a cell in which the horizontal distance of the cathode and the anode from the barrier is varied in accordance with the effective lengths of the anode and the cathode below the level of the barrier and the upper portion of the anode is above the level of the top portion of the cathode and is partially or wholly below the surface of the electrolyte characterised in that the horizontal distance between the anode and the cathode is from inch to 1.5 inch and in that at least the lower extremity of the barrier is distanced, measured horizontally, by not more than inch from the anode and preferably in that at least the lower extremity of the barrier approaches to Within a distance, measured horizontally, of inch to inch from the anode and no portion approaches more nearly than inch.

The gas permeable carbon anode, it is stated, has to have at least a gas permeability such that no free fluorine bubbles are liberated into alia at the operating current density.

It is also stated that preferably the permeability is approximately 30.

In accordance with a preferred embodiment of the present invention the process for the production of fluorine by electrolysis at a temperature of -110 C., preferably 80-85 C., from a fused electrolyte comprising a substantially dry mixture of potassium fluoride and hydrogen fluoride having a composition approximating substantially to KF.1.8HF to KF.2.2HF under nonpolarization conditions and so that the composition of the mixture is maintained substantially at KF.1.8HF to KF.2.2,HF at a current efiiciency of greater than and preferably greater than with no evolution of fluorine as free bubbles at a substantially vertical solid gas permeable carbon anode of substantially uniform cross section in conjunction with a gas impermeable barrier which entirely surrounds but is not in contact with the anode by employing current desities of for example 0.1 to 1.1 amperes per square inch and for instance eflie'ctive lengths of anode and cathode up to for example 18 inches and in a cell in which the horizontal distance of the cathode and the anode from the barrier is varied in accordance with the effective lengths of the anode and the cathode below the level of the barrier and the upper portion of the anode is above the level of the top portion of the cathode and is partially or wholly below the surface of the electrolyte and in which the body is of electro-conducting material and in which the cathode is in metallic electrical connection with said body comprises arranging that the horizontal distance between the anode and the cathode is between 0.5 inch and 1.5 inch and for at least the lower extremity of the barrier to be distanced, measured horizontally, by not more than inch from the anode and preferably for at least the lower extremity of the barrier to approach to within a distance, measured horizontally of A inch to inch from the anode and for no portion to approach more nearly than A inch, employing a cathode which permits water to be circulated therethrough and while electrolysis is taking place circulating water therethrough.

Should the electrolyte contain any substantial quantity of, for example, nickel salts, for instance 30-100 p.p.m. nickel salts, expressed as nickel, for instance derived from nickel-containing components of the electrolytic cell, it would appear for this preferred embodiment of the invention to be carried out successfully that the electrolyte should also contain substantial traces of moisture.

It would also seem for this preferred embodiment that it is often desirable for the electrolyte to contain traces of moisture or nickel salts if the resulting fluorine is to be of low hydrofluoric acid content.

For the process of the invention in general it would seem that when nickel salts are present in the electrolyte the distance between the anode and cathode should pref erably be between 1.25 and 1.5 inches and that the distance of the lower extremity of the barrier from the anode has to be so chosen that an undesirably large proportion of fluorine does not enter the cathode compartmerit and an undesirably large proportion of hydrogen does not enter the anode compartment of the cell.

It would appear that nickel salts when present in the electrolyte in for instance the aforesaid concentrations give rise during the electrolysis of the electrolyte to a higher concentration of nickel salts at the anode surface.

The relationship, in other Words, between the evolution of fluorine bubbles and the presence of nickel salts and/ or Water in the fused electrolyte potassium fluoride/ hydrogen fluoride of the specified composition appears to be as follows:

(1) If a nickel salt is present in the absence of water some of the fluorine is likely to break away from the carbon anode as free bubbles and may escape the barrier and enter the hydrogen compartment. The barrier must there'- before be positioned sufliciently far from the anode to catch them. On the other hand the barrier must notbe so near to the cathode as to permit hydrogen to enter the anode compartment.

2) If a nickel salt is absent then substantially no' free fluorine bubbles leave the anode and the barrier may be as close A inch from the anode.

(3) If a nickel salt is present together with sufficient water then substantially no free bubbles of fluorine are present and the barrier may be as close as inch from the anode.

(4) When substantially no free fluorine bubbles are present the current efliciency may even approach. 100% but the hydrogen fluoride in the fluorine may be high.

The gas permeable carbon anode has to have at least a gas permeability such that no free fluorine bubbles are liberated inter alia at the operating current density. Ordi-- nary electrode carbon which has such a low permeability as about 0.05 when determined in accordance with the procedure described in co-pending application Ser. No. 784,151 is unsuitable for the carrying out of the present invention and is considered to be impermeable for the purposes of the present invention.

Preferably the permeability is approximately 30.

The term effective anode length refers to that portion of the anode length which is below the gas barrier and is opposite to the elfective length of the cathode.

Current density is determined with reference to that portion of the anode surface which is directly opposite the cathode.

The term polarization is used herein to denote the condition under which at a fixed voltage a sudden or gradual decrease occurs in the current flowing through the cell to a value which is a small fraction of that pass ing when the cell is operating normally. The eifect may be temporary or permanent and is essentially an anodic phenomenon.

When in the production of fluorine according to the process of the invention by electrolysis at a temperature of -105 (3., and preferably 80-85 C., the fused mixture of potassium fluoride and hydrogen fluoride having a composition approximating substantially to KF.1.8 HF to KF.2.2 HP is substantially dry and is substantially free from, e.g. nickel salts, or contains a substantial quantity of nickel salts and substantial traces of moisture, or contains traces of moisture then one form of an electrolytic cell adapted for the carrying out of said process comprises a body of electro-c-onducting material containing therein the fluorine-containing liquid electrolyte, a substantially vertical solid gas permeable carbon anode of substantially uniform cross section and for instance of a gas permeability between 1.0 and 30 surrounded at its upper portion by a gas impermeable barrier dipping below the surface of the electrolyte and at its lower portion by a wholly submerged cathode, as for instance in the form of a tubular coil, adapted to permit cooling liquid to pass therethrough and which is below said barrier wherein the body is preferably provided with means serving for heating and cooling its contents, wherein the horizontal distance between the anode and the cathode is in the range of 0.5 to 1.5 inches, wherein at least the lower extremity of the barrier approaches to within a distance (measured horizontally) from the anode which is in the range inch to A; inch and no portion approaches more nearly than A inch, and wherein the cathode is preferably in metallic electrical connection with said body.

The dimension of the anode/barrier gap is related to the electrode separation which it is proposed to use. Thus a small electrode separation requires a small anode/barrier gap though any electrode separation above the minimum referred to herein may be used with the minimum anode gap quoted.

If, however, in the process of the invention said 'fused mixture is'substantially dry and contains substantial quantities of for instance nickel salts then said form of elec' trolytic 'cellis modified so that the horizontal distance between the anode and the cathode is between 1.25 and 1.5 inches and the distance between at least the lower extremity of the barrier and the anode is not greater than 7 inch.

Preferably, when the anode barrier separation is in the minimum range i.e. when at least the lower extremity of the barrier is within a distance from. the anode of A to /8 inch the barrier in the aforesaid form of an electrolytic cell has a downwardly and inwardly projecting slope or curve so shaped as to direct any hydrogen that rises from the cathode and comes into contact with the 'barrier upwardly and outwardly away from the anode.

The cathode should, preferably, as indicated in the aforesaid form of electrolytic cell, be wholly below the gas barrier. extremity of the cathode from the lower extremity of the barrier is not critical but must be adequate to allow of ready escape of hydrogen liberated in the space between anode and cathode. Clearly if this vertical separation is inadequate, then when high current density is employed there is a possibility of a particularly brisk evolution of hydrogen leading to crowding of hydrogen bubbles within this space, so increasing the danger of hydrogen finding its Way into the anode compartment. The requisite vertical separation of cathode and barrier is thus influenced not only by the spatial relationship of anode, cathode and barrier but also by the material of the anode and the current density at which the cell is to be operated. This vertical separation is not, however, a factor of major importance and a convenient practice is to make it of comparable magnitude with the electrode separation.

The anode, cathode and barrier may be cylindrical in form although other shapes, for instance of rectangular or square section, or even of hexagonal section, may be used if desired.

When working with such small clearances between the electrodes and the barrier as are specified above, robust and accurate construction, particularly of the gas barrier, is of considerable importance. One suitable type of barrier that may be employed when the anode is of cylindrical cross-section is a hollow cylinder made of a suitable metal with an inturned flange, which is neither horizontal nor sloping upwards towards the anode, at the lower extremity extending inwards for such distance as to leave a clearance from the anode of the desired size. The anode/barrier gap is in this case measured from the inner circumference of the flange to the face of the anode. Obviously the barrier may, if desired, be a simple hollow cylinder; the flanged structure has, however, the advantage of greater rigidity which is obviously important when such small clearances are used. A flanged construction or one with a tapered or inturned lower extremity can be more reliably and precisely positioned with respect to the anode than a barrier which is only, say inch, from the anode for its full length. Also not only is such an arrangement more resistant to deformation that would cause short-circuiting, but it virtually limits to very small dimensions the possible areas of contact that could be involved in short-circuiting between anode and barrier.

The body of the cell (i.e. the container), the gas barrier and the cathode may all conveniently be made from mild steel, although other materials resistant to the electrolyte and the products of electrolysis may be used if desired. For instance, the barrier may be made of Monel or nickel.

The anode may be a simple block of carbon, whose minimum transverse dimension is at least 1 /2 inches, preferably 2 inches. For, for instance, 60-amp. cells it may be convenient to use cylindrical anodes of diameter up to 3 inches; for IO-amp. cells, 2 inches is a convenient value. The length of the anode block is not of major importance. The point to be borne in mind in this connection is simply that if this total length be unduly great there Will be an appreciable voltage drop as one proceeds down the anode from the lead-in conductor at the top towards the lower extremity, so that the eflective potential difference between anode and cathode will not be as great in the lower portion of the cell as in the upper portion. The barrier is conveniently made 4 inches to 8 inches deep but can be greater or smaller if desired. It must dip sufiiciently below the electrolyte surface to make an adequate liquid seal at the base of the anode (or fluorine) compartmenta depth of immersion of 2 inches is convenient. The length of the The vertical distance separating the upper anode block that is opposite to, and so in operative relationship with, the cathode, i.e. the effective lengths of the anode and cathode, is of more importance. We have used lengths ranging from 2. inches to 14 /2 inches and find little difference in their effect apart from the voltage drop due to the ohmic resistance alluded to above. However, an unduly long and narrow interelectrode space can more easily leadto crowding of hydrogen bubbles if a high current density is used and also an increased length of carbon anode requires greater consideration to be given to its fragility.

The electrode separation, the depth of the interelectrode space, the current density employed, the material of which the gas permeable carbon anode is constructed and the size of the anode gap are all interconnected, but once the relevant principles are appreciated the appropriate adjustment of these various factors is a routine matter easily within the competence of the operator skilled in this art.

It is to be noted that the maximum current density which can be used will be dictated by the current density at which polarisation occurs and/ or, in the absence of nickel salts or in the presence of nickel salts and water, at which break-away of fluorine bubbles takes place and/ or, in the presence of nickel salts at which substantial break-away of fluorine bubbles takes place. Alternatively or additionally the maximum current density is also dictated by the maximum permissible temperature of the carbon anode.

Commercially available electrode carbons having a gas permeability of 25-30 and 10 respectively are eminently suitable.

High current density, a long narrow interelectrode space and an inadequate vertical clearance between the lower extremity of the barrier and the upper extremity of the cathode all tend to produce hydrogen bubble crowding, but this can be avoided without increasing the electrode separation by increasing the vertical cathode-barrier clearance, shortening the electrodes and diminishing the anode gap.

By adjusting these various factors appropriately as indicated, we have been able to construct cells having electrode separations and anode/barrier gaps of the surprisingly small dimensions defined above which have given continuous trouble-free operation for periods as long as 9 months, and this without any need for electrode replacement or removal of sludge from the cell. The current efliciency was 97-98% using current densities up to 1.1 amp/sq. in.

The saving achieved in a standard 60-amp cell consequent on reducing the electrode separation from the 2 inches hitherto employed to /2 inch with an 11 inch anode (i.e. below the bottom of the barrier) was 1.2 volts when operating at a curent density of 0.6 amp./ sq. inch and 1.9 volts when working at a current density of 0.9 amp./ sq. inch.

The saving for an approx. 1400 amp. cell consequent on reducing the electrode separation from 2 /2 inches to 1% inches with an 8 inch anode (i.e. below the bottom of the barrier) was 1.4 volts when operating at a current density of 0.5 amp./ sq. inch and 2.5 volts when working at a current density of 0.9 amp./ sq. inch.

From the data recorded in the following table wherein two laboratory cells of nominal 200 amps. capacity with different electrode separation and no water-cooling of the cathodes and therefore not in accordance with the present invention are compared with two works cells with different electrode separation and water-cooled cathodes and therefore in accordance with the present invention, it is seen that the works cells, even with the larger electrode separation and therefore with greater production of heat on working, operate with less hydrofluoric acid in the fluorine.

Laboratory Cell Works Cell Laboratory Cell Works Cell N o. of anodes Size of anodes (in Type of cooling None inside the cell. Finned Water cooled cathodes.

Electrode separation (in.) Load (21.) Voltage (v.) Anode current density (amp/m?) Heat generated per anode (w.) Hydrgfluoric acid in fluorine, v./v. percen Equivalent anode temperature at 41% HF C . J Measured anode temperature l2 13% x 11 x 2%-.-

Water cooled cathodes.

1 13% x 11 x 2%. None inside From the above table it is seen that there is reasonably good agreement between the calculated and the actual anode temperatures for the two cells where comparative figures are available. From this it is concluded that the actual temperatures of the anodes in the other two cells may be taken as being close to the caluclated temperatures. It is therefore held that the actual temperature of the works cell with the 1.25 inch anode/cathode separation at 89 C. (actual)97 C. (calculated) is lower than that of the cell with the 2.5 inch anode/cathode separation at 106 C. (calculated).

In addition, it will be noted, the amount of heat produced per anode in the 1.25 inch anode/cathode separation works cell is about the same as that produced in the laboratory cell with the 2 inch anode/cathode separation and yet the anode temperatures are 89-97 C. and 140-145 C. respectively. This illustrates the beneficial influence of decreased electrode separation combined With water-cooled cathode in the works cell. This beneficial influence is also shown by the lower hydrofluoric acid content of the fluorine from the works cell with the 1.25 inch anode/cathode separation.

The more detailed practice of the invention is illustrated by the following description.

One form of cell suitable for carrying out the invention is illustrated in the diagrammatic drawing accompanying the provisional specification which represents a vertical section through the said cell. Referring to the drawing, 1 is a container of mild steel or other suitably resistant metal, surrounded by a heating jacket 2 adapted for water, steam or electrical heating (water shown), and preferably thermostatically controlled. 3 is the lid and 4 is the cover of the container 1 and 5 is a carbon anode. The lid 3 is insulated from the cover 4 and from the carbon anode S by insulating material 6. 7 is a gas seal between the cover 4 and the container 1. The carbon anode 5 is partly submerged in electrolyte 8. An electrically conducting rod 9 is connected to the carbon anode 5 and is insulated from the cell lid 3 by the insulating material 6. In close proximity to the carbon anode 5 is a cathode 10 in the form of a helical tubular coil. The cathode 10 may be of mild steel, copper or other material substantially resistant to the electrolyte 8 and products of electrolysis. The helical tubular coil 10 may be produced from tubular material of 0.75 inch external diameter and 0.5 inch internal diameter. The cathode 10 passes through and is in electrical contact with the container 1. Attached to the lid 3 is a gas impermeable barrier 11 which surrounds that part of the carbon anode 5 above the level of the top of the cathode 10. Pipe 12 is for fluorine take-off and is connected through the cell lid 3 to the space between the carbon anode '5 and the gas barrier 11. Pipe 13 is for take-01f of hydrogen and passes through the cover 4 of the con tainer 1. Pipe 14 is for addition of hydrofluoric acid and passes through the cover 4 of the container 1 into the electrolyte 8.

In comparative experiments it has been ascertained that a works cell which has a cell temperature which starts to rise from above C. and approaches C. at a load of 850 amperes when the cathodes are not water cooled, that is to say when the cell is not being used according to the process of the invention, has a cell temperature of 83-85 C. at a load of approximately 1700 amperes when the cathodes are water cooled, that is to say when the cell is being used according to the process of the invention.

What we claim is:

1. A method of producing fluorine by electrolysis in a cell having an electrically-conductive body, at a substantially vertical gas-permeable carbon anode, the upper portion of which is surrounded by, but out of contact with, a gas impermeable barrier and the lower portion of 'Which is adjacent a cathode, which comprises placing in the cell an electrolyte composition containing about 1 part potassium fluoride and about 1.8 to 2.2 parts hydrogen fluoride, maintaining the composition at a temperature of 80 to C., passing electric current between the anode and the cathode and internally cooling said cathode by passing cooling medium therethrough.

2. A method of producing fluorine as set forth in claim 1 including maintaining electrical contact between the electrically-conductive cell body and the cathode.

3. A method of producing fluorine as set forth in claim 1 in which the cooling medium is Water.

4. A method of producing fluorine as set forth in claim 3 in which the electrolyte is maintained at a temperature of 80 to 85 C.

5. A method of producing fluorine as set forth in claim 3 in which the electric current density is between about 0.1 and 1.1 amperes per square inch.

6. A method of producing fluorine as set forth in claim 3 in which the electrolyte contains nickel salts and substantial traces of water.

7. A method of producing fluorine as set forth in claim 3 in which the electrolyte contains traces of moisture.

8. A method of producing fluorine as set forth in claim 3 in which the electrolyte is substantialy dry and contains nickel salts.

References Cited in the file of this patent UNITED STATES PATENTS 2,515,614 Schumacher July 18, 1950 2,534,638 Swinehart Dec. 19, 1954 2,684,940 Rudge et al. July 27, 1954 2,693,445 Howell et al. Nov. 2, 1954 

1. A METHOD OF PRODUCING FLUORINE BY ELECTROLYSIS IN A CELL HAVING AN ELECTRICALLY-CONDUCTIVE BODY, AT A SUBSTANTIALLY VERTICAL GAS-PERMEABLE CARBON ANODE, THE UPPER PORTION OF WHICH IS SURROUNDED BY, BUT OUT OF CONTACT WITH, A GAS IMPERMEABLE BARRIER AND THE LOWER PORTION OF WHICH IS ADJACENT A CATHODE, WHICH COMPRISES PLACING IN THE CELL AN ELECTROLYTE COMPOSITION CONTAINING ABOUT 1 PART POTASSIUM FLUORIDE AND ABOUT 1.8 TO 2.2 PARTS HYDROGEN FLUORIDE, MAINTAINING THE COMPOSITION AT A TEMPERATURE OF 80 TO 110*C., PASSING ELECTRIC CURRENT BETWEEN THE ANODE AND THE CATHODE AND INTERNALLY COOLING SAID CATHODE BY PASSING COOLING MEDIUM THERETHROUGH. 